An intruder detection system is commonly provided with a light receiving window having a light transmissive pane (hereinafter referred to as "window-pane") through which an acceptor receives infrared ray radiated by an intruder passing through a detection area. It may happen that an intruder who knows the presence of a detection system deliberately covers the window with a non-transmissive material such as cloth so as to disable the detection system. Such an obstruction may inadvertently happen owing to wind or any other cause. As a result, an alarm will fail, an automatic door will not open, or no signal is transmitted to a contract security service.
In order to obviate an obstruction to the detection system, Japanese Patent Publication Kokai No. 4-190500 discloses an intruder detection system which is additionally provided with a light emitter spaced from the window. The light emitter projects light at intervals. The detection system also incorporates a checker for receiving the light from the light emitter in synchronism with the emission of light. This system is disadvantageous in requiring extra equipment such as a light-emitter and a checker, thereby complicating the structure and increasing the production cost.
Japanese Patent Publication Kokai No. 2-287278 teaches the provision of a light emitter adjacent to a light acceptor within the window frame so as to know the presence of a probable obstacle by detecting any increase in the light reflection upon the obstacle. However, if the cover is black cloth or any other light absorptive material, this anti-obstruction system will not function because of detecting no increase in the reflecting light.
A further proposal identified by International Publication No. WO 88/03301 discloses a burglar sensor having a light emitter provided outside the window so as to monitor any increase in a reflecting light upon a probable cover or caused by any other obstructive act. This device has the same disadvantage as the last-mentioned system, in that if the used cover is black, the amount of the reflecting light do not increase, thereby failing to detect the presence of malicious covering.
There can be two kinds of obstruction; one is to cover the window directly with a non-transmissive object, and the other is to place an obstructive covering between the light emitter and the light acceptor, at some distance from the window.